Apparatus for reducing the impact of program disturb

ABSTRACT

The unintentional programming of an unselected (or inhibited) non-volatile storage element during a program operation that intends to program another non-volatile storage element is referred to as “program disturb.” A system is proposed for programming and/or reading non-volatile storage that reduces the effect of program disturb. In one embodiment, different verify levels are used for a particular word line (or other grouping of storage elements) during a programming process. In another embodiment, different compare levels are used for a particular word (or other grouping of storage elements) during a read process.

CLAIM OF PRIORITY

This application is a divisional application of U.S. patent applicationSer. No. 12,725,446, “Apparatus for Reducing the Impact of ProgramDisturb,” filed on Jan. 16, 2010, which is a divisional application ofU.S. patent application Ser. No. 12,365,648, “Apparatus for Reducing theImpact of Program Disturb,” filed on Feb. 4, 2009, which is a divisionalapplication of U.S. Pat. No. 7,499,326, issued Mar. 3, 2009, applicationSer. No. 11/413,683, filed Apr. 28, 2006, titled “Apparatus for Reducingthe Impact of Program Disturb,” which claims the benefit of U.S.Provisional Application No. 60/791,365, “Reducing The Impact Of ProgramDisturb For A Word Line,” Inventor Gerrit Jan Hemink, filed on Apr. 12,2006. All of the above-identified applications are incorporated hereinby reference in their entirety.

CROSS-REFERENCE TO RELATED APPLICATIONS

This Application is related to the following United States PatentApplications, all of which are incorporated herein by reference in theirentirety:

U.S. patent application Ser. No. 11/414,758, filed Apr. 28, 2006, nowU.S. Pat. No. 7,436,713, “Reducing The Impact of Program Disturb,” byGerrit Jan Hemink, Attorney Docket No. SAND-01104US4;

U.S. patent application Ser. No. 11/413,671, filed Apr. 28, 2006, nowU.S. Pat. No. 7,426,137, “Apparatus for Reducing The Impact of ProgramDisturb During Read,” by Gerrit Jan Hemink; Attorney Docket No.SAND-01104US3; and

U.S. patent application Ser. No. 11/413,951, filed Apr. 28, 2006, nowU.S. Pat. No. 7,515,463, “Reducing The Impact Of Program Disturb DuringRead,” by Gerrit Jan Hemink, Attorney Docket No. SAND-01104US1.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a non-volatile memory.

2. Description of the Related Art

Semiconductor memory devices have become more popular for use in variouselectronic devices. For example, non-volatile semiconductor memory isused in cellular telephones, digital cameras, personal digitalassistants, mobile computing devices, non-mobile computing devices andother devices. Electrical Erasable Programmable Read Only Memory(EEPROM) and flash memory are among the most popular non-volatilesemiconductor memories.

Many types of EEPROM and flash memories utilize a floating gate that ispositioned above and insulated from a channel region in a semiconductorsubstrate. The floating gate is positioned between source and drainregions. A control gate is provided over and insulated from the floatinggate. The threshold voltage of the transistor is controlled by theamount of charge that is retained on the floating gate. That is, theminimum amount of voltage that must be applied to the control gatebefore the transistor is turned on to permit conduction between itssource and drain is controlled by the level of charge on the floatinggate.

One example of a flash memory system uses the NAND structure, whichincludes arranging multiple transistors in series, sandwiched betweentwo select gates. The transistors in series and the select gates arereferred to as a NAND string. FIG. 1 is a top view showing one NANDstring. FIG. 2 is an equivalent circuit thereof. The NAND stringdepicted in FIGS. 1 and 2 includes four transistors 100, 102, 104 and106 in series and sandwiched between a first (or drain) select gate 120and a second (or source) select gate 122. Select gate 120 connects theNAND string to a bit line via bit line contact 126. Select gate 122connects the NAND string to source line 128. Select gate 120 iscontrolled by applying the appropriate voltages to select line SGD.Select gate 122 is controlled by applying the appropriate voltages toselect line SGS. Each of the transistors 100, 102, 104 and 106 has acontrol gate and a floating gate. For example, transistor 100 hascontrol gate 100CG and floating gate 100FG. Transistor 102 includescontrol gate 102CG and a floating gate 102FG. Transistor 104 includescontrol gate 104CG and floating gate 104FG. Transistor 106 includes acontrol gate 106CG and a floating gate 106FG. Control gate 100CG isconnected to word line WL3, control gate 102CG is connected to word lineWL2, control gate 104CG is connected to word line WL1, and control gate106CG is connected to word line WL0.

Note that although FIGS. 1 and 2 shows four memory cells in the NANDstring, the use of four transistors is only provided as an example. ANAND string can have less than four memory cells or more than fourmemory cells. For example, some NAND strings will include eight memorycells, 16 memory cells, 32 memory cells, 64 memory cells, etc. Thediscussion herein is not limited to any particular number of memorycells in a NAND string.

A typical architecture for a flash memory system using a NAND structurewill include several NAND strings. For example, FIG. 3 shows three NANDstrings 202, 204 and 206 of a memory array having many more NANDstrings. Each of the NAND strings of FIG. 3 includes two selecttransistors (also called gates) and four memory cells. For example, NANDstring 202 includes select transistors 220 and 230, and memory cells222, 224, 226 and 228. NAND string 204 includes select transistors 240and 250, and memory cells 242, 244, 246 and 248. Each NAND string isconnected to the source line by its source select gate (e.g. selecttransistor 230 and select transistor 250). A selection line SGS is usedto control the source select gates (e.g., 230 and 250).

The various NAND strings are connected to respective bit lines by selecttransistors 220, 240, etc., which are controlled by select line SGD.Each bit line and the respective NAND string(s) that are connected tothat bit line via a bit line contact comprise the columns of the arrayof memory cells. Bit lines are shared with multiple NAND strings.Typically, the bit line runs on top of the NAND strings in a directionperpendicular to the word lines and is connected to one or more senseamplifiers.

The word lines (WL3, WL2, WL1 and WL0) comprise the rows of the array.Word line WL3 is connected to the control gates for memory cell 222 andmemory cell 242. Word line WL2 is connected to the control gates formemory cell 224, memory cell 244 and memory cell 252. Word line WL1 isconnected to the control gates for memory cell 226 and memory cell 246.Word line WL0 is connected to the control gates for memory cell 228 andmemory cell 248.

Each memory cell can store data (analog or digital). When storing onebit of digital data (referred to as a binary memory cell), the range ofpossible threshold voltages of the memory cell is divided into tworanges which are assigned logical data “1” and “0.” In one example of aNAND type flash memory, the voltage threshold is negative after thememory cell is erased, and defined as logic “1.” The threshold voltageafter programming is positive and defined as logic “0.” When thethreshold voltage is negative and a read is attempted by applying 0volts to the control gate, the memory cell will turn on to indicatelogic one is being stored. When the threshold voltage is positive and aread operation is attempted by applying 0 volts to the control gate, thememory cell will not turn on, which indicates that logic zero is stored.

A memory cell can also store multiple levels of information (referred toas a multi-state memory cell). In the case of storing multiple levels ofdata, the range of possible threshold voltages is divided into thenumber of levels of data. For example, if four levels of information isstored, there will be four threshold voltage ranges assigned to the datavalues “11”, “10”, “01”, and “00.” In one example of a NAND type memory,the threshold voltage after an erase operation is negative and definedas “11”. Positive threshold voltages are used for the states of “10”,“01”, and “00.”

Relevant examples of NAND type flash memories and their operation areprovided in the following U.S. Patents/Patent Applications, all of whichare incorporated herein by reference: U.S. Pat. No. 5,570,315; U.S. Pat.No. 5,774,397; U.S. Pat. No. 6,046,935; U.S. Pat. No. 6,456,528; andU.S. Pat. Publication No. US2003/0002348. The discussion herein can alsoapply to other types of flash memory in addition to NAND as well asother types of non-volatile memory.

When programming a flash memory cell, a program voltage is applied tothe control gate and the bit line is grounded. Due to the voltagedifferential between the channel of the flash memory cell and thefloating gate, electrons from the channel area under the floating gateare injected into the floating gate. When electrons accumulate in thefloating gate, the floating gate becomes negatively charged and thethreshold voltage of the memory cell is raised. To apply the programvoltage to the control gate of the cell being programmed, that programvoltage is applied on the appropriate word line. As discussed above,that word line is also connected to one memory cell in each of the otherNAND strings that utilize the same word line. For example, whenprogramming memory cell 224 of FIG. 3, the program voltage will also beapplied to the control gate of memory cell 244 because both memory cellsshare the same word line. A problem arises when it's desired to programone cell on a word line without programming other cells connected to thesame word line, for example, when it's desired to program memory cell224 and not memory cell 244. Because the program voltage is applied toall memory cells connected to a word line, an unselected memory cell (amemory cell that is not to be programmed) on the same word line maybecome inadvertently programmed. For example, memory cell 244 isadjacent to memory cell 224. When programming memory cell 224, there isa concern that memory cell 244 might unintentionally be programmed. Theunintentional programming of the unselected memory cell on the selectedword line is referred to as “program disturb.”

Several techniques can be employed to prevent program disturb. In onemethod known as “self boosting,” the unselected NAND strings areelectrically isolated from the corresponding bit lines and a passvoltage (e.g. 7-10 volts, but not limited to this range) is applied tothe unselected word lines during programming. The unselected word linescouple to the channel area of the unselected NAND strings, causing avoltage (e.g., 6-10 volts) to exist in the channel of the unselectedNAND strings, thereby reducing program disturb. Self boosting causes aboosted voltage to exist in the channel which lowers the voltagedifferential across the tunnel oxide and hence reduces program disturb.Note that the boosted channel voltage can vary largely since the boostedchannel voltage depends on the value of the pass voltage and also on thestate of the memory cells, with boosting being most efficient (highestchannel voltage) when all memory cells in the NAND string are in theerased state.

FIGS. 4 and 5 depict NAND strings that are being programmed andinhibited using the self-boosting method. FIG. 4 depicts a NAND stringbeing programmed. The NAND string of FIG. 4 includes eight memory cells304, 306, 308, 310, 312, 314, 316 and 318. Each of those eight memorycells includes a floating gate (FG) and a control gate (CG). Betweeneach of the floating gates are source/drain regions 330. In someimplementations, there is a P-type substrate (e.g., Silicon), an N-wellwithin the substrate and a P-well within the N-well (all of which arenot depicted to make the drawings more readable). Note that the P-wellmay contain a so called channel implantation that is usually a P-typeimplantation that determines or helps to determine the threshold voltageand other characteristics of the memory cells. The source/drain regions330 are N+ diffusion regions that are formed in the P-well.

At one end of the NAND string is a drain side select gate 324. The drainselect gate 324 connects the NAND string to the corresponding bit linevia bit line contact 334. At another end of the NAND string is a sourceselect gate 322. Source select gate 322 connects the NAND string tocommon source line 332. During programming, the memory cell selected forprogramming (e.g., memory cell 312) receives a program voltage Vpgm onits associated word line. The program voltage Vpgm can typically varybetween 12 to 24 volts. In one embodiment, the program voltage signal isa set of pulses which increase in magnitude with each new pulse. A passvoltage Vpass of approximately 8 volts is applied to the control gatesof the memory cells that are not selected for programming. Source selectgate 322 is in an isolating state, receiving 0 volts at its gate (G). Alow voltage is applied to the common source line 332. This low voltagecan be zero volts. However, the source voltage can also be slightlyhigher than zero volts to provide better isolation characteristics ofthe source side select gate. A voltage Vsgd, which is typically in therange of the power supply voltage Vdd (e.g., 2.5 volts), is applied todrain side select gate 324. Zero volts is applied to bit line contact334 via the corresponding bit line to enable programming of the selectedmemory cell 312. Channel 340 is at or close to zero volts. Because ofthe voltage differential between the channel and the floating gate ofmemory cell 314, electrons tunnel through the gate oxide (also commonlyreferred to as tunnel oxide) into the floating gate by Fowler-Nordheimtunneling.

The NAND string of FIG. 5 depicts a NAND string being inhibited fromprogramming. The NAND string includes eight memory cells 350, 352, 354,356, 358, 360, 362 and 364. The NAND string also includes drain selectgate 366 connecting the NAND string to the corresponding bit line viabit line contact 374, and source select gate for 368 connecting the NANDstring to common source line 332. Between each of the floating gatestacks are source/drain regions 370. The NAND string of FIG. 5 has Vsgdapplied to the gate of the drain select gate 366, zero volts applied tothe gate of the source side select gate 368 and zero volts (or aslightly higher voltage) at the common source line 332. Bit line contact374 receives the power supply voltage Vdd via the corresponding bit linein order to inhibit the programming of memory cell 358.

When Vdd is applied, the drain select transistor 366 will initially bein a conducting state; therefore, the channel area under the NAND stringwill partly be charged up to a higher potential (higher than zero voltsand typically equal or almost equal to Vdd). This charging is commonlyreferred to as pre-charging. The pre-charging will stop automaticallywhen the channel potential has reached Vdd or a lower potential given byVsgd-Vt, where Vt equals the threshold voltage of the drain select gate366. In general, during pre-charging, Vsgd is chosen in such a way thatVsgd-Vt>Vdd so that the channel area under the NAND string can bepre-charged to Vdd. After the channel has reached that potential, theselect gate transistor is non-conducting or made non-conducting bylowering Vsgd to a value similar to Vdd (e.g. 2.5 volts). Subsequently,the voltages Vpass and Vpgm are ramped up from zero volts to theirrespective final values (not necessarily at the same time), and becausethe drain side select gate transistor 366 is in a non-conducting state,the channel potential will start to rise because of the capacitivecoupling between the word lines and the channel area. This phenomenon iscalled self boosting. It can be seen from FIG. 5 that channel 380 isboosted, more or less uniformly, to a boosting voltage. Because thevoltage differential between the floating gate of memory cell 358 andchannel 380 has been reduced, programming is inhibited. More informationabout programming NAND flash memory, including self boosting techniques,can be found in U.S. Pat. No. 6,859,397, “Source Side Self BoostingTechnique for Non-Volatile Memory,” Lutze at al., incorporated herein byreference in its entirety.

Another attempt to address program disturb is Erased Area Self Boosting(“EASB”). EASB attempts to isolate the channel of previously programmedcells from the channel of the cell being inhibited. In the EASB method,the channel area of the selected NAND string is divided into two areas.An area at the source side of the selected word line that can contain anumber of programmed (or erased cells) memory cells and an area at thedrain side of the selected word line in which the cells are still in theerased state, or at least not yet in the final programmed state. The twoareas are separated by a word line that is biased to a low voltage,typically zero volts. Because of this separation, the two areas can beboosted to different potentials. In almost all cases, the area at thedrain side of the selected word line will be boosted to a higherpotential than the area at the source side. Since the highest boostedarea is the area with the erased cells, this boosting method is referredto as Erased Area Self Boosting (EASB).

Although the above boosting methods have reduced program disturb, theyhave not eliminated the problem. One effect that can occur to the memorycell next to the source select gate (e.g., memory cell 350 is next tosource select gate 368 of FIG. 5) is Gate Induced Drain Leakage (GIDL),which is also referred to as Band-To-Band-Tunneling. GIDL causes thegeneration of electrons at the source select gate when the channel underthe NAND string is inhibited from programming (boosted to a highvoltage). Subsequently, the generated electrons are accelerated in thestrong lateral electric field towards the floating gate of the memorycell next to the source select gate. Some of the electrons can gainsufficient energy to be injected into the tunnel oxide under thefloating gate or in the floating gate itself and, thus modify thethreshold voltage of the corresponding memory cell.

FIG. 6 shows a portion of the NAND string of FIG. 5, with a zooming-inon the drain and a portion of the channel for memory cell 350. Due toboosting of the NAND string during a program inhibit operation (forexample when other NAND strings are being programmed), a high voltage ispresent in the channel area of the boosted NAND string (see boostedchannel 380). This high voltage is also present at the junction areabetween source select gate 368, which is typically biased at 0V, andmemory cell 350 next to source select gate 368. This bias condition maycause GIDL, which can result in the creation of electron hole pairs. Theholes will go to P-well area 384. The electrons will move to the boostedchannel area 380. In general, there is a lateral electric field presentin the junction area between the source select gate and the memory cellnext to the source side select gate because part of that junction(drain/source) is depleted due to the large voltage difference betweenchannel area under the memory cells and the channel area under theselect gate. The electrons can be accelerated in the electric field andmay gain enough energy to be injected in the tunnel oxide of the memorycell next to the source side select gate or may even reach the floatinggate of that memory cell. In both cases, the threshold voltage of thecorresponding memory cell will change due to the presence of theinjected electrons, thereby, risking an error when reading the memorycell next to the source select gate.

Thus, there is a need for a new mechanism to reduce the impact ofprogram disturb.

SUMMARY OF THE INVENTION

A system is proposed for programming and/or reading non-volatile storageelements that reduces the effect of program disturb. In one set ofimplementations, different verify levels are used for a particular wordline (or other grouping of storage elements) during a programmingprocess. For one set of examples that use multi-state devices, thetarget level for one programmed state, two programmed states, anothersubset of programmed states, or all of the programmed states can bedifferent. In some embodiments, different pages of data associated withthe particular word line (or other grouping of storage elements) coulduse different (one, two, another subset or all) target levels. In otherembodiments, different storage elements associated with the particularword line (or other grouping of storage elements) could use different(one, two, another, subset or all) target levels. In one embodiment, theword line that receives the different target levels is chosen based onits position relative to a boosted region.

One embodiment includes programming a group of non-volatile storageelements using a group of target levels and programming a particular setof non-volatile storage elements using a particular set of target levelsso that threshold distributions of the particular set of non-volatilestorage elements are within corresponding threshold distributions of thegroup of non-volatile storage elements upon completion of a programmingprocess. At least one of the particular set of target levels is below acorresponding target level of the group of target levels.

One embodiment includes programming a group of non-volatile storageelements using a group of target levels and programming a particular setof non-volatile storage elements using a particular set of target levelsso that threshold voltage distributions of the particular set ofnon-volatile storage elements are shifted to at least protrude less(including not protruding at all) from corresponding threshold voltagedistributions of the group of non-volatile storage elements uponcompletion of a programming process, at least one of the particular setof target levels is below a corresponding target level of the group oftarget levels.

One embodiment includes programming a set of one or more non-volatilestorage elements using a group of target levels and programming aparticular non-volatile storage element using a particular set of targetlevels. At least one of the particular set of target levels is below acorresponding target level of the group of target levels. The particularnon-volatile storage element is adjacent to a source select gate.

One embodiment includes programming a first set of one or morenon-volatile storage elements using a first set of target levels andprogramming a second set of one or more non-volatile storage elementsusing a second set of target levels after programming the first set ofone or more non-volatile storage elements. The first set of one or morenon-volatile storage elements are programmed first. The first set of oneor more non-volatile storage elements are connected to a first word lineand the second set of one or more non-volatile storage elements areconnected to a set of word lines programmed after the first word lineduring a sequence of programming operations. The second set of targetlevels is different than the first set of target levels.

A system is proposed for programming and/or reading non-volatile storagethat reduces the effect of program disturb. In one set ofimplementations, different read compare values are used for a particularword line (or other grouping of storage elements) during a read process.The word line that will receive the different read compare values ischosen based on position of the word line with respect to the positionof a boosting region during the programming process.

One embodiment includes reading a first set of non-volatile storageelements using a first set of read compare values and reading a secondset of one or more non-volatile storage elements using a second set ofread compare values. The first set of non-volatile storage elements areconnected to a first control line. The second set of non-volatilestorage elements are connected to a second set of control linesdifferent than the first control line. At least one of the first set ofread compare values is different than a corresponding compare level ofthe second set of read compare values. In one example of animplementation (but not all implementations), the first control line isnext to a source select control line.

One embodiment includes programming a first set of non-volatile storageelements using a first set of target levels and programming a second setof one or more non-volatile storage elements using the same first set oftarget levels. The first set of non-volatile storage elements isassociated with a first control line. The first control line is adjacentto a second control line. The process also includes providing aprogramming signal on the first control line and providing a signal onthe second control line requiring non-volatile storage elementsconnected to the second control line to turn off in response to thesignal. The second set of one or more non-volatile storage elements areassociated with a set of control lines. The first control line and thesecond control line are not in the set of control lines. The first setof one or more non-volatile storage elements are read using a first setof read compare values. The second set of one or more non-volatilestorage elements are read using a second set read compare values. Atleast one of the first set of read compare values is different than acorresponding compare level of the second set of read compare values.

The various methods described herein can be performed by variousdevices. One example of a suitable apparatus includes non-volatilestorage elements and a managing circuit in communication with thenon-volatile storage elements. The non-volatile storage elements includea first set of non-volatile storage elements and a second set ofnon-volatile storage elements. The managing circuit performs the variousmethods described herein in relation to the first set of non-volatilestorage elements and the second set of non-volatile storage elements. Inone embodiment, the managing circuit includes any one or a combinationof a controller, a state machine, command circuits, control circuits anddecoders. In other embodiments, the managing circuit can also includeother elements suitable for the particular implementations.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top view of a NAND string.

FIG. 2 is an equivalent circuit diagram of the NAND string.

FIG. 3 is a schematic diagram depicting three NAND strings.

FIG. 4 shows a NAND string being programmed.

FIG. 5 shows a NAND string being inhibited, using a self boostingmethod.

FIG. 6 depicts a portion of a NAND string.

FIG. 7 is a block diagram of one example of a memory system.

FIG. 8 illustrates an example of an organization of a memory array.

FIG. 9 depicts a set of threshold voltage distributions.

FIGS. 10A, 10B and 10C depict threshold voltage distributions.

FIG. 11 is a flow chart describing one embodiment of a process forprogramming and reading non-volatile memory.

FIGS. 12A and 12B depict threshold voltage distributions.

FIG. 13 is a flow chart describing one embodiment of a process forprogramming and reading non-volatile memory.

FIG. 14 is a flow chart describing one embodiment of a process forprogramming and reading non-volatile memory.

FIG. 15 is a flow chart describing one embodiment of a process forprogramming and reading non-volatile memory.

FIG. 16 is a flow chart describing one embodiment of a programmingoperation.

FIG. 17 is a signal diagram depicting one embodiment of a readoperation.

FIG. 18 is a flow chart describing one embodiment of a process forprogramming and reading non-volatile memory.

FIG. 19 depicts a set of threshold voltage distributions.

FIG. 20 is a flow chart describing one embodiment of a process forprogramming and reading non-volatile memory.

DETAILED DESCRIPTION

FIG. 7 is a block diagram of one embodiment of a flash memory systemthat can be used to implement one or more embodiments described herein.Other systems and implementations can also be used. Memory cell array502 is controlled by column control circuit 504, row control circuit506, p-well control circuit 508 and c-source control circuit 510. Columncontrol circuit 504 is connected to the bit lines of memory cell array502 for reading data stored in the memory cells, for determining a stateof the memory cells during a program operation, and for controllingpotential levels of the bit lines to promote or inhibit programming anderasing. Row control circuit 506 is connected to the word lines toselect one of the word lines, to apply read voltages, to apply programvoltages combined with the bit line potential levels controlled bycolumn control circuit 504, and to apply an erase voltage. In oneembodiment, row control 306 and column control 304 include decoders toselect the appropriate word lines and bit lines. C-source controlcircuit 510 controls a common source line (labeled as “C-source” in FIG.8) connected to the memory cells. P-well control circuit 508 controlsthe p-well voltage.

The data stored in the memory cells is read out by the column controlcircuit 504 and is output to external I/O lines via data input/outputbuffer 512. Program data to be stored in the memory cells is input tothe data input/output buffer 512 via the external I/O lines, andtransferred to the column control circuit 504. The external I/O linesare connected to controller 518.

Command data for controlling the flash memory device is input tocontroller 518. The command data informs the flash memory of whatoperation is requested. The input command is transferred to statemachine 516 which is part of control circuitry 515. State machine 516controls column control circuit 504, row control circuit 506, c-sourcecontrol 510, p-well control circuit 508 and data input/output buffer512. State machine 516 can also output status data of the flash memorysuch as READY/BUSY or PASS/FAIL.

Controller 518 is connected to or connectable with a host system such asa personal computer, a digital camera, or personal digital assistant,etc. It communicates with the host that initiates commands, such as tostore or read data to or from the memory array 502, and provides orreceives such data. Controller 518 converts such commands into commandsignals that can be interpreted and executed by command circuits 514which are part of control circuitry 515. Command circuits 514 are incommunication with state machine 516. Controller 518 typically containsbuffer memory for the user data being written to or read from the memoryarray.

One exemplary memory system comprises one integrated circuit chip thatincludes controller 518, and one or more integrated circuit chips thateach contain a memory array and associated control, input/output andstate machine circuits. The memory arrays and controller circuits of asystem can be integrated on one or more integrated circuit chips. Thememory system may be embedded as part of the host system, or may beincluded in a memory card (or other package) that is removably insertedinto the host systems. Such a card may include the entire memory system(e.g. including the controller) or just the memory array(s) withassociated peripheral circuits (with the controller or control functionbeing embedded in the host). Thus, the controller can be embedded in thehost or included within the removable memory system.

In some implementations, some of the components of FIG. 7 can becombined. In various designs, one or more of the components of FIG. 7(alone or in combination), other than memory cell array 502, can bethought of as a managing circuit. For example, a managing circuit mayinclude any one of or a combination of control circuitry 515, commandcircuits 514, state machine 516, column control circuit 504, row controlcircuit 506, p-well control circuit 508, c-source control circuit 510and data I/O 512.

With reference to FIG. 8, an exemplary structure of memory cell array502 is described. As one example, a NAND flash EEPROM is described thatis partitioned into 1,024 blocks. The data stored in each block can besimultaneously erased. In one embodiment, the block is the minimum unitof memory cells that are simultaneously erased. In each block, in thisexample, there are 8,512 columns. Each block is typically divided into anumber of pages which can be a unit of programming. Other units of datafor programming are also possible. In one embodiment, individual pagesmay be divided into segments and the segments may contain the fewestnumber of cells that are written at one time as a basic programmingoperation. One or more pages of data are typically stored in one row ofmemory cells.

In each block of the example in FIG. 8 there are 8,512 columns that aredivided into even bit lines (BLe) and odd bit lines (BLo). In anodd/even bit line architecture, memory cells along a common word lineand connected to the odd bit lines are programmed at one time, whilememory cells along a common word line and connected to even bit linesare programmed at another time. FIG. 8 shows four memory cells connectedin series to form a NAND string. Although four cells are shown to beincluded in each NAND string, more or less than four can be used (e.g.,16, 32, or another number). One terminal of the NAND string is connectedto a corresponding bit line via a drain select gate (connected to selectgate drain line SGD), and another terminal is connected to c-source viaa source select gate (connected to select gate source line SGS).

In other embodiments, the bit lines are not divided into odd and evenbit lines. Such architectures are commonly referred to as all bit linearchitectures. In an all bit line architecture, all the bit lines of ablock can be simultaneously selected during read and program operations.Memory cells along a common word line and connected to any bit line canbe programmed at the same time.

In another embodiment the bit lines are divided into planes. Forexample, there can be a left plane (left most 4256 bit lines) and aright plane (right most 4256 bit lines). Each plane can be programmedseparately or both planes can be programmed at the same time. In someembodiments, there can be more than two planes. Other arrangements canalso be used.

During read and programming operations of one embodiment using anodd/even bit line architecture, 4,256 memory cells are simultaneouslyselected. The memory cells selected have the same word line (e.g.WL2-i), and the same kind of bit line (e.g. even bit lines). Therefore,532 bytes of data can be read or programmed simultaneously. These 532bytes of data that are simultaneously read or programmed form a logicalpage. Therefore, in this example, one block can store at least eightpages. When each memory cell stores two bits of data (e.g. a multi-statecell), one block stores 16 pages. Other sized blocks and pages can alsobe used. Additionally, architectures other than that of FIGS. 7 and 8can also be used to implement embodiments.

In the read and verify operations, the select gates of a selected blockare raised to one or more select voltages and the unselected word lines(e.g., WL0, WL1 and WL3) of the selected block are raised to a read passvoltage (e.g. 4.5 volts) to make the transistors operate as pass gates.The selected word line of the selected block (e.g., WL2) is connected toa reference voltage, a level of which is specified for each read andverify operation in order to determine whether a threshold voltage ofthe concerned memory cell is above or below such level. For example, ina read operation of a binary memory cell, the selected word line WL2 isgrounded, so that it is detected whether the threshold voltage is higherthan 0V. In a verify operation of a binary memory cell, the selectedword line WL2 is connected to 0.8V, for example, so that as programmingprogresses it is verified whether or not the threshold voltage hasreached the target level of 0.8V. The source and p-well are at zerovolts during read and verify. The selected bit lines (BLe) arepre-charged to a level of, for example, 0.7V. If the threshold voltageis higher than the read or verify level, the potential level of theconcerned bit line (BLe) maintains the high level, because of theassociated non-conductive memory cell. On the other hand, if thethreshold voltage is lower than the read or verify level, the potentiallevel of the concerned bit line (BLe) decreases to a low level, forexample less than 0.5V, because of the conductive memory cell. The stateof the memory cell is detected by a sense amplifier that is connected tothe bit line and senses the resulting bit line voltage. The differencebetween whether the memory cell is programmed or erased depends onwhether or not net negative charge is stored in the floating gate. Forexample, if negative charge is stored in the floating gate, thethreshold voltage becomes higher and the transistor can be inenhancement mode of operation. In another embodiment, a memory cell canbe read by detecting the current conducted by a memory cell.

When programming a memory cell in one example, the drain and the p-wellreceive 0 volts while the control gate receives a series of programmingpulses with increasing magnitudes. In one embodiment, the magnitudes ofthe pulses in the series range from 12 volts to 24 volts. In otherembodiments, the range of pulses in the series can be different. Themagnitude of the pulses is increased with each pulse by a predeterminedstep size. In one embodiment that includes the memory cells storingmultiple bits of data, an exemplary step size is 0.2 volts (or 0.4volts). During programming of memory cells, verify operations arecarried out in the periods between the programming pulses. The cellsthat are verified to be sufficiently programmed are locked out, forexample in NAND cells, by raising the bit line voltage from 0 toV_(DD)(e.g., 2.5 volts) for all subsequent programming pulses toterminate the programming process for those memory cells.

At the end of a successful program process, the threshold voltages ofthe memory cells should be within one or more distributions of thresholdvoltages for programmed memory cells or within a distribution ofthreshold voltages for erased memory cells, as appropriate. FIG. 9illustrates example threshold voltage distributions for the memory cellarray when each memory cell stores two bits of data. FIG. 9 shows afirst threshold voltage distribution E for erased memory cells. Threethreshold voltage distributions, A, B and C for programmed memory cells,are also depicted. In one embodiment, the threshold voltages in the Edistribution (also referred to as physical state E) are negative and thethreshold voltages in the A, B and C distributions (also referred to asphysical states A, B and C) are positive.

Each distinct threshold voltage range of FIG. 9 corresponds topredetermined values for the set of data bits. The specific relationshipbetween the data programmed into the memory cells and the thresholdvoltage levels of the memory cells depends upon the data encoding schemeadopted for the memory cells. For example, U.S. Pat. No. 6,222,762 andU.S. Patent Publication 2004/0255090, both of which are incorporatedherein by reference in their entirety, describe various data encodingschemes for multi-state flash memory cells. In one embodiment, datavalues are assigned to the threshold voltage ranges using a Gray codeassignment so that if the threshold voltage of a floating gateerroneously shifts to its neighboring physical state, only one bit willbe affected. One example assigns “11” to threshold voltage range E(state E), “10” to threshold voltage range A (state A), “00” tothreshold voltage range B (state B) and “01” to threshold voltage rangeC (state C). However, in other embodiments, Gray code is not used.Although FIG. 9 shows four states, the present invention can also beused with other multi-state structures including those that include moreor less than four states.

FIG. 9 shows three read compare values—voltages Vra, Vrb and Vrc—forreading data from memory cells. By testing whether the threshold voltageof a given memory cell is above or below Vra, Vrb and Vrc, the systemcan determine what state the memory cell is in.

FIG. 11 shows three verify target levels—voltages Vva2, Vvb2 and Vvc2.When programming memory cells to state A, the system will test whetherthose memory cells have a threshold voltage greater than or equal toVva2. When programming memory cells to state B, the system will testwhether the memory cells have threshold voltages greater than or equalto Vvb2. When programming memory cells to state C, the system willdetermine whether memory cells have their threshold voltage greater thanor equal to Vvc2.

In one embodiment, known as full sequence programming, memory cells canbe programmed from the erase state E directly to any of the programmedstates A, B or C. While some memory cells are being programmed fromstate E to state A, other memory cells are being programmed from state Eto state B and/or from state E to state C.

In addition to the full sequence programming discussed above, FIG. 9also illustrates an example of a two-pass technique of programming amulti-state memory cell that stores data for two different pages: alower page and an upper page. For state E, both pages store a “1.” Forstate A, the lower page stores a “0” and the upper page stores a “1.”For state B, both pages store “0.” For state C, the lower page stores“1” and the upper page stores “0.” Note that although specific bitpatterns have been assigned to each of the states, different bitpatterns may also be assigned.

In a first programming pass, the memory cell's threshold voltage levelis set according to the bit to be programmed into the lower logicalpage. If that bit is a logic “1,” the threshold voltage is not changedsince it is in the appropriate state as a result of having been earliererased. However, if the bit to be programmed is a logic “0,” thethreshold level of the cell is increased to be state A, as shown byarrow 600. That concludes the first programming pass.

In a second programming pass, the memory cell's threshold voltage levelis set according to the bit being programmed into the upper logicalpage. If the upper logical page bit is to store a logic “1,” then noprogramming occurs since the cell is in one of the states E or A,depending upon the programming of the lower page bit, both of whichcarry an upper page bit of “1.” If the upper page bit is to be a logic“0,” then the threshold voltage is shifted. If the first pass resultedin the cell remaining in the erased state E, then in the second phasethe cell is programmed so that the threshold voltage is increased to bewithin state C, as depicted by arrow 604. If the cell had beenprogrammed into state A as a result of the first programming pass, thenthe memory cell is further programmed in the second pass so that thethreshold voltage is increased to be within state B, as depicted byarrow 602. The result of the second pass is to program the cell into thestate designated to store a logic “0” for the upper page withoutchanging the data for the lower page.

In one embodiment, a system can be set up to perform full sequencewriting if enough data is written to fill up an entire page. If notenough data is written for a full page, then the programming process canprogram the lower page programming with the data received. Whensubsequent data is received, the system will then program the upperpage. In yet another embodiment, the system can start writing in themode that programs the lower page and convert to full sequenceprogramming mode if enough data is subsequently received to fill up anentire (or most of a) word line's memory cells. More details of such anembodiment are disclosed in U.S. Patent Application titled “PipelinedProgramming of Non-Volatile Memories Using Early Data,” Ser. No.11/013,125, filed on Dec. 14, 2004, inventors Sergy AnatolievichGorobets and Yan Li, incorporated herein by reference in its entirety.

FIGS. 10A-C disclose another process for programming non-volatile memorythat reduces the effect of floating gate to floating gate coupling by,for any particular memory cell, writing to that particular memory cellwith respect to a particular page subsequent to writing to adjacentmemory cells for previous pages. In one example of an implementation ofthe process taught by FIGS. 10A-C, the non-volatile memory cells storetwo bits of data per memory cell, using four data states. For example,assume that state E is the erased state and states A, B and C are theprogrammed states. State E stores data 11. State A stores data 01. StateB stores data 10. State C stores data 00. This is an example of non-Graycoding because both bits change between adjacent states A & B. Otherencodings of data to physical data states can also be used. Each memorycell stores two pages of data. For reference purposes these pages ofdata will be called upper page and lower page; however, they can begiven other labels. With reference to state A for the process of FIGS.10A-C, the upper page data is 0 and the lower page data is 1. Withreference to state B, the upper page data is 1 and the lower page datais 0. With reference to state C, both pages store data 0.

The programming process of FIGS. 10A-C is a two-step process. In thefirst step, the lower page is programmed. If the lower page is to remaindata 1, then the memory cell state remains at state E. If the data is tobe programmed to 0, then the threshold voltage of the memory cell israised such that the memory cell is programmed to state B′. FIG. 10Atherefore shows the programming of memory cells from state E to stateB′. State B′ depicted in FIG. 10A is an interim state B; therefore, theverify target level is depicted as Vvb2′, which is lower than Vvb2.

In one embodiment, after a memory cell is programmed from state E tostate B′, its neighbor memory cell in the NAND string will then beprogrammed with respect to its lower page. For example, looking back atFIG. 2, after the lower page for memory cell 106 is programmed, thelower page for memory cell 104 would be programmed. After programmingmemory cell 104, the floating gate to floating gate coupling effect willraise the apparent threshold voltage of memory cell 106 if memory cell104 had a threshold voltage raised from state E to state B′. This willhave the effect of widening the threshold voltage distribution for stateB′ to that depicted as threshold voltage distribution 620 of FIG. 10B.This apparent widening of the threshold voltage distribution will beremedied when programming the upper page.

FIG. 10C depicts the process of programming the upper page. If thememory cell is in erased state E and the upper page is to remain at 1,then the memory cell will remain in state E. If the memory cell is instate E and its upper page data is to be programmed to 0, then thethreshold voltage of the memory cell will be raised so that the memorycell is in state A. If the memory cell was in intermediate thresholdvoltage distribution 620 and the upper page data is to remain at 1, thenthe memory cell will be programmed to final state B. If the memory cellis in intermediate threshold voltage distribution 620 and the upper pagedata is to become data 0, then the threshold voltage of the memory cellwill be raised so that the memory cell is in state C. The processdepicted by FIGS. 10A-C reduces the effect of floating gate to floatinggate coupling because the maximum Vt shift during the upper pageprogramming is reduced as cells are programmed from the E state to Astate or from the B′ state to the C-state instead of from the E to Cstate in FIG. 9, as a result, the upper page programming of neighbormemory cells will have less effect on the apparent threshold voltage ofa given memory cell. Although FIGS. 10A-C provide an example withrespect to four data states and two pages of data, the concepts taughtby FIGS. 10A-C can be applied to other implementations with more or lessthan four states and more or less than two pages.

Note that there are various orders for programming the various pages.The current invention can be used with many different orders forprogramming the various pages, as suitable for the particularimplementation. More information on programming can be found in U.S.patent application. Ser. No. 11/099,133, “Compensating for CouplingDuring Read Operations of Non-Volatile Memory,” filed on Apr. 5, 2005 byJian Chen, incorporated herein by reference in its entirety.

In order to reduce the effect of program disturb, different targetverify levels can be used for a particular word line (or other groupingof storage elements) during the verify step(s) of a programming process.FIG. 11 provides a flow chart explaining one embodiment of a processthat uses one set of target levels for the word line next to the sourceselect gates and source select line, and another set of target levelsfor the other word lines. In step 650 of FIG. 11, word line WL0, thefirst word line to be programmed and the word line next to the sourceselect line SGS (see FIGS. 2, 3 and 8) is programmed using a first setof target levels. That is, all or a subset of the memory cells connectedto WL0 are programmed using the first set of target levels to verifywhether the respective memory cells have completed programming. In step652, the remaining (or another subset of) word lines are programmedusing a second set of target levels. That is, for example, all or asubset of the memory cells connected to WL1-WL4 are programmed using thesecond set of target levels to verify whether the respective memorycells have completed programming. Looking back at FIG. 5, word line WL0is at the edge of boosting regions 380.

In one embodiment, the first set of target levels includes Vva1, Vvb1and Vvc1; and the second set of target levels includes Vva2, Vvb2 andVvc2. In one example implementation, Vva1 is in the order of 100 to 200mV lower than the corresponding Vva2, Vvb1 is in the order of 100 to 200mV lower than the corresponding Vvb2, and Vvc1 is in the order of 100 to200 mV lower than the corresponding Vvc2.

In other embodiments, a subset of the first set of target levels can bethe same as a subset of the second set of target level. That is becausein some implementations, it may be found that program disturb is only aproblem for memory cells in some of the programmed states (e.g., stateA, or states A and B). Therefore, in some embodiments, the first set oftarget levels includes Vva1, Vvb2 and Vvc2 and the second set of targetlevels includes Vva2, Vvb2 and Vvc2. In other embodiments, the first setof target levels includes Vva1, Vvb1 and Vvc2 and the second set oftarget levels includes Vva2, Vvb2 and Vvc2. Other permutations can alsobe implemented.

The target levels of the first and second set of target levels are thecompare points used during the programming process to decide when amemory cell has finished programming. For example, a memory cell on WL0intended to be programmed to state A will finish the programming processwhen its threshold voltage has reached Vva1 and a memory cell on WL3intended to be programmed to state B will finish the programming processwhen its threshold voltage has reached Vvb2.

There are some devices that program using a two phased coarse/fineprogramming methodology. The first phase, a coarse programming phase,includes attempts to raise the threshold voltage in a faster manner andpaying relatively less attention to achieving a tight thresholddistribution. The second phase, a fine programming phase, attempts toraise the threshold voltage in a slower manner in order to reach thetarget threshold voltage while also achieving a tighter thresholddistribution. Example of coarse/fine programming methodologies can befound in the following patent documents that are incorporated herein byreference in their entirety: U.S. Patent Publication 2005/0162916 U.S.Pat. No. 6,301,161; U.S. Pat. No. 5,712,815; U.S. Pat. No. 5,220,531;and U.S. Pat. No. 5,761,222. When verifying a memory cell duringprogramming, some prior solutions will first perform the verify processfor the coarse mode using an intermediate verify level and thensubsequently perform the verify process for the fine mode using thetarget level for verification. The present solution for changing thetarget levels applies to the target level for verification during thefine mode. In some cases, the intermediate verify level can also bechanged.

By lowering target levels for word line WL0, the threshold voltages ofthe programmed memory cells connected to WL0 should, over the populationof memory cells, be lowered in comparison to what they would be if thetarget levels were not lowered. For example, FIG. 12A shows twothreshold distributions for one programmed state (e.g., state A).Distribution 670 represents one example of a threshold voltagedistribution (# of memory cells versus threshold voltage) of memorycells connected to all word lines except word line WL0. Distribution 672represents one example of a threshold voltage distribution of memorycells connected to word line WL0 when all of the word lines use the sametarget levels. Because of the program disturb described above,distribution 672 is shifted to the right and widened as compared todistribution 670. As can be seen, distribution 672 protrudes out fromdistribution 670 at the upper end.

By using a lower set of target levels for word line WL0, as per steps650 and 652, the threshold voltage distributions for the programmedmemory cells associated with WL0 are shifted to the left so that theyare within the corresponding threshold voltage distributions associatedwith the other word lines at the completion of a programming process.For example, FIG. 12B shows distribution 672 shifted to the left afterthe completion of a programming process (one or many pages programmed toaccommodate the storage of a set of data, such as a digital picture orother file) as a result of using lower target levels for WL0 so thatdistribution 672 fits inside distribution 670. In another embodiment,distribution 672 is shifted to the left after the completion of aprogramming process (one or many pages programmed to accommodate thestorage of a set of data, such as a digital picture or other file) as aresult of using lower target levels for WL0 so that distribution 672 atleast protrudes out less from distribution 670 at the upper end ascompared to the depiction in FIG. 12A. If distribution 672 at leastprotrudes out less from distribution 670 at the upper end as compared tothe depiction in FIG. 12A, then any remaining errors may be fixed duringa read process (e.g. using ECC).

Note that distribution 672 and distribution 670 represent one state(e.g., state A, state B, state C, or a different state). In an example,with three programmed states (plus an erased state), there would be upto three pairs of corresponding threshold voltage distributions similarto distribution 672 and distribution 670. In embodiments whichcontemplate storing more than two bits of information in a memory cell,more than three pairs of threshold voltage distributions similar todistribution 672 and distribution 670 may exist. In some cases, lessthan all states will have their distribution shifted on WL0.

Looking back at FIG. 11, in step 660 all or a subset of the memory cellsconnected to WL0 are read using a set of read compare values. Forexample, the read compare values Vra, Vrb and Vrc (see FIG. 9) can beused to read the data stored in the memory cells. In step 662, all or asubset of the memory cells connected to the word lines other than WL0are read using the same set of read compare values as used in step 660.Note that the arrow between step 652 and 660 is dashed to represent thatstep 660 may be performed at a different time than step 650 and 652and/or in a manner unrelated to steps 651 and 652.

FIG. 13 provides a flow chart explaining an embodiment of a process thatuses one set of target levels for a word line at the edge of a boostingregion and another set of target levels for the other word lines. In theprocess of FIG. 11 described above, WL0 is at the edge of the boostingregion. However, in other embodiments the edge of a boosting region canbe located elsewhere. In step 680 of FIG. 13, memory cells connected toa first set of word lines are programmed using the second set of targetlevels to verify whether the respective memory cells have completedprogramming. In step 682, memory cells connected to the word line at theedge of the boosting region are programmed using the first set of targetlevels to verify whether the respective memory cells have completedprogramming. In step 684, memory cells connected to the remaining wordlines are programmed using the second set of target levels to verifywhether the respective memory cells have completed programming. In otherembodiments, additional sets of word lines and additional edge wordlines can be included.

In step 690, memory cells connected to the first set of word lines areread using a set of read compare values (e.g. Vra, Vrb and Vrc). In step692, memory cells connected to the word line at the edge of the boostingregion during programming are read using the same set of read comparevalues as used in step 692. In step 694, memory cells connected to theremaining set of word lines are read using the same set of read comparevalues as used in step 692. Note that the arrow between step 684 and 690is dashed to represent that step 690 may performed at a different timethan step 684 and/or in a manner unrelated to step 684.

FIG. 14 provides a flow chart explaining an embodiment of a process thatuses different sets of target levels for different pages of dataassociated with a word line at the edge of a boosting region (e.g., nextto source select line) and another set of target levels for the otherword lines. In step 710, memory cells connected to word line WL0 (oranother word line) are programmed using the first set of target levels(Vva1, Vvb1, Vvc1) for a first page of data and a different set oftarget levels for a second page of data. For example, the different setof target levels can include target levels Vva3, Vvb3, Vvc3, where:

Vva3≠Vva1, Vvb3≠Vvb1, Vvc3≠Vvc1; and   (1)

Vva3<Vva2, Vvb3<Vvb2, Vvc3<Vvc2.   (2)

In other embodiments, the different set of target levels can includesome of the target levels Vva3, Vva3 and Vvb3, with the remaining targetlevels being the same as the first set of target levels or the secondset of target levels.

In step 712, the memory cells connected to additional word lines otherthan WL0 are programmed using the second set of target levels, asdescribed above.

In step 720, all or a subset of the memory cells connected to the wordline WL0 are read using a set of read compare values (e.g. Vra, Vrb andVrc). In step 722, all or a subset of the memory cells connected to theword lines other than the word line WL0 are read using the same set ofread compare values as used in step 720. Note that the arrow betweenstep 712 and 720 is dashed to represent that step 720 may performed at adifferent time than step 712 and 710 and/or in a manner unrelated tosteps 712 and 710.

FIG. 15 provides a flow chart explaining an embodiment of a process thatuses different sets of target levels for different planes (or section orgroups) of memory cells connected to a word line at the edge of aboosting region (e.g., next to the source select line, such as WL0) andanother set of target levels for the other word lines. In step 740,memory cells connected to word line WL0 (or a different word line) areprogrammed using the first set of target levels for a first grouping ofmemory cells and the different set of target levels for a secondgrouping of memory cells. In step 742, the memory cells connected toadditional word lines other than WL0 are programmed using the second setof target levels.

For example, looking back at FIG. 8, the first plane or grouping caninclude memory cells on NAND strings connected to bit lines Ble0-Ble2127and the second plane or grouping can include memory cells on NANDstrings connected to bit lines Ble2128-Ble4255. In another embodiment,the first plane or grouping can include memory cells on NAND stringsconnected to bit lines Blo0-Blo2127 and the second plane or grouping caninclude memory cells on NAND strings connected to bit linesBlo2128-Blo4255. Another alternative is for the first plane or groupingto include memory cells on NAND strings connected to bit lines 0 through(½(x)-1) and the second plane or grouping can include memory cells onNAND strings connected to bit lines ½(x) through (x-1), where x is thetotal number of bit lines for user data. Another alternative is for thefirst plane or grouping to include memory cells on NAND stringsconnected to odd bit lines and the second plane or grouping can includememory cells on NAND strings connected to even bit lines. Othergroupings can also be used, and more than two groupings can be used.

In step 750, all or a subset of the memory cells connected to the wordline WL0 (or another word line) are read using a set of read comparevalues (e.g. Vra, Vrb and Vrc). In step 752, all or a subset of thememory cells connected to the word lines other than word line WL0 areread using the same set of read compare values as used in step 750. Notethat the arrow between step 742 and 750 is dashed to represent that step750 may be performed at a different time than steps 740 and 742 and/orin a manner unrelated to steps 740 and 742.

FIG. 16 is a flow chart describing one embodiment of a programmingoperation. The process of FIG. 16 can be used to program the memorycells connected to a word line using full sequence programming. Inembodiments that program different pages at different times, the processof FIG. 16 can be used to program one page or one pass for a particularword line or set of memory cells. Because a programming process mayinclude programming multiple pages and memory cells connected tomultiple word lines, a programming process may include performing theprogramming operation of FIG. 16 multiple times.

The memory cells to be programmed are erased at step 840. Step 840 caninclude erasing more memory cells than those to be programmed (e.g., inblocks or other units). At step 842, soft programming is performed tonarrow the distribution of erased threshold voltages for the erasedmemory cells. Some memory cells may be in a deeper erased state thannecessary as a result of the erase process. Soft programming can applysmall programming pulses to move the threshold voltage of the erasedmemory cells closer to the erase verify level. At step 850 of FIG. 16, a“data load” command is issued by controller 518 and input to commandcircuits 514, allowing data to be input to data input/output buffer 512.The input data is recognized as a command and latched by state machine516 via a command latch signal, not illustrated, input to commandcircuits 514. At step 852, address data designating the page address isinput to row controller or decoder 506 from the controller or host. Theinput data is recognized as the page address and latched via statemachine 516, affected by the address latch signal input to commandcircuits 514. At step 854, a page of program data for the addressed pageis input to data input/output buffer 512 for programming. For example,582 bytes of data could be input in one embodiment. That data is latchedin the appropriate registers for the selected bit lines. In someembodiments, the data is also latched in a second register for theselected bit lines to be used for verify operations. At step 856, a“program” command is issued by controller 518 and input to datainput/output buffer 512. The command is latched by state machine 516 viathe command latch signal input to command circuits 514.

Triggered by the “program” command, the data latched in step 854 will beprogrammed into the selected memory cells controlled by state machine516 using the stepped pulses applied to the appropriate word line. Atstep 858, Vpgm, the programming voltage pulse applied to the selectedword line, is initialized to the starting magnitude (e.g., ˜12V oranother suitable level) and a program counter PC maintained by statemachine 516 is initialized at 0. At step 860, the first Vpgm pulse isapplied to the selected word line. If logic “0” is stored in aparticular data latch indicating that the corresponding memory cellshould be programmed, then the corresponding bit line is grounded. Onthe other hand, if logic “1” is stored in the particular latchindicating that the corresponding memory cell should remain in itscurrent data state, then the corresponding bit line is connected toV_(DD) to inhibit programming. As depicted in FIGS. 4 and 5, theunselected word lines receive Vpass, the drain select signal is set atVsgd and the source select signal is set at 0 v. The source line is setat or near zero volts.

At step 862, the states of the selected memory cells are verified usingthe appropriate set of target levels, as discussed above with respect toFIGS. 11, 13, 14 and 15. If it is detected that the threshold voltage ofa selected cell has reached the appropriate target level, then the datastored in the corresponding data latch is changed to a logic “1.” If itis detected that the threshold voltage has not reached the appropriatetarget level, the data stored in the corresponding data latch is notchanged. In this manner, a bit line having a logic “1” stored in itscorresponding data latch does not need to be programmed. When all of thedata latches are storing logic “1,” the state machine knows that allselected cells have been programmed. At step 864, it is checked whetherall of the data latches are storing logic “1.” If so, the programmingprocess is complete and successful because all selected memory cellswere programmed and verified to their target states. A status of “PASS”is reported at step 866. Note that in some implementations, at step 864it is checked whether at least a predetermined number of data latchesare storing a logic “1”. This predetermined number can be less than thenumber of all data latches. Thus allowing the programming process tostop before all memory cells have reached their appropriate verifylevels. The memory cells that are not successfully programmed can becorrected using error correction during the read process.

If, at step 864, it is determined that not all of the data latches arestoring logic “1,” then the programming process continues. At step 868,the program counter PC is checked against a program limit value. Oneexample of a program limit value is 20; however, other values can beused in various implementations. If the program counter PC is not lessthan 20, then it is determined at step 869 whether the number of memorycells that have not been successfully programmed is equal to or lessthan a predetermined number. If the number of unsuccessfully programmedmemory cells is equal to or less than the predetermined number, then theprogramming process is flagged as passed and a status of PASS isreported at step 871. In many cases, the memory cells that are notsuccessfully programmed can be corrected using error correction duringthe read process. If however, the number of unsuccessfully programmedmemory cells is greater than the predetermined number, the programprocess is flagged as failed and a status of FAIL is reported at step870. If the program counter PC is less than 20, then the Vpgm level isincreased by the step size (e.g., 0.2-0.4 volt step size) and theprogram counter PC is incremented at step 872. After step 872, theprocess loops back to step 860 to apply the next Vpgm pulse.

FIG. 17 is a timing diagram depicting the behavior of various signalsduring one iteration of a read or verify operation. For example, if thememory cells are binary memory cells, the process of FIG. 17 may beperformed in parallel for each memory cell during a read or verifyprocess. If the memory cells are multi-state memory cells with fourstates (e.g., E, A, B, and C), the process of FIG. 17 may be performedthree times for each memory cell in parallel during a read or verifyprocess. For example, when reading data from a memory cell with fourstates, the read process may be performed once with Vcgr=Vra, once withVcgr=Vrb and once with Vcgr=Vrc, or other sets of read values. Whenverifying data for a memory cell with four states, the read process maybe performed once with Vcgr=Vva2, once with Vcgr=Vvb2 and once withVcgr=Vvc2, or other sets of verify levels.

In general, during the read and verify operations, the selected wordline is connected to a voltage, a level of which is specified for eachread and verify operation in order to determine whether a thresholdvoltage of the concerned memory cell has reached such level. Afterapplying the word line voltage, the conduction current of the memorycell is measured to determine whether the memory cell turned on inresponse to the voltage applied to the word line. If the conductioncurrent is measured to be greater than a certain value, then it isassumed that the memory cell turned on and the voltage applied to theword line is greater than the threshold voltage of the memory cell. Ifthe conduction current is not measured to be greater than the certainvalue, then it is assumed that the memory cell did not turn on and thevoltage applied to the word line is not greater than the thresholdvoltage of the memory cell.

There are many ways to measure the conduction current of a memory cellduring a read or verify operation. In one example, the conductioncurrent of a memory cell is measured by the rate it discharges adedicated capacitor in the sense amplifier. In one embodiment, a memoryarray that uses all bit line programming can measure the conductioncurrent of a memory cell by the rate it discharges a dedicated capacitorin the sense amplifier. In another example, the conduction current ofthe selected memory cell allows (or fails to allow) the NAND string thatincludes the memory cell to discharge the bit line. The charge on thebit line is measured after a period of time to see whether it has beendischarged or not. In one embodiment, a memory array that uses odd/evenprogramming can measure the conduction current of a memory cell bydetermining whether the bit line has discharged.

FIG. 17 shows signals SGD, WL_unsel. WLn, SGS, Selected BL, and Sourcestarting at Vss (approximately 0 volts). SGD represents the signal onthe drain select line connected to the gate of the drain select gate.SGS represents the signal on the source select line connected to thegate of the source select gate. WLn is the word line selected forreading/verification. WL_unsel represents the unselected word lines.Selected_BL is the bit line selected for reading/verification. Source isthe source line for the memory cells.

FIG. 17 describes the behavior of a system that measures the conductioncurrent of a memory cell by determining whether the bit line hasappropriately discharged. At time t1 of FIG. 10, SGD is raised to Vdd(e.g., approximately 3.5 volts) or another voltage, typically in the3-5V range, the unselected word lines (WL_unsel) are raised to Vread(e.g., approximately 5.5 volts), the selected word line WLn is raised toVcgr, and the selected bit line Selected BL is precharged toapproximately 0.7 volts. The voltage Vread acts as pass a voltagebecause it causes the unselected memory cells to turn on and act as passgates. At time t2, the source select gate is turned on by raising SGS toVdd. This provides a path to reduce the charge on the bit line. If thethreshold voltage of the memory cell selected for reading is greaterthan Vcgr, then the selected memory cell will not turn on (or at leastwill not turn on sufficiently) and the bit line will not discharge (orat least will not discharge sufficiently), as depicted by signal line890. If the threshold voltage in the memory cell selected for reading isbelow Vcgr, then the memory cell selected for reading will turn on(conduct) and the bit line voltage will reduce, as depicted by curve892. At some point after time t2 and prior to time t3 (as determined bythe particular implementation), the appropriate sense amplifier willdetermine whether the bit line voltage has reduced a sufficient amount.At time t3, the depicted signals will be lowered to Vss (or anothervalue for standby or recovery). Note that in other embodiments, thetiming of some of the signals can be changed.

Rather than (or in addition to) reducing the impact of program disturbby changing the target levels for verification, different read comparevalues can be used for a particular word line (or other grouping ofstorage elements) during a read process. For example, FIG. 18 describesone embodiment of a process for operating non-volatile storage thatincludes using different read compare values for one or more particularword lines. In step 920, the memory cells connected to WL0 areprogrammed using the second set of target levels mentioned above. Instep 922, the memory cells connected to the additional word lines areprogrammed using the second set of target levels mentioned above. Instep 930, the memory cells connected to WL0 are read using a first setof read compare values. That is, the second set of read compare valuesare applied to the appropriate control gates via the appropriate wordlines. In step 932, the memory cells connected to the additional wordlines are read using a second set of read compare values. Note that thearrow between step 922 and 930 is dashed to represent that step 930 mayperformed at a different time than step 922 and 920 and/or in a mannerunrelated to steps 922 and 920.

In another embodiment, steps 920 and 930 can be applied to multiple wordlines (e.g., WL0 and WL1, WL0-2, or other groupings including groupingsthat do not include WL0 but may be at an edge, as explained below). Thatis, there could be two groups of word lines. The first group of wordlines would be read using the first set of read compare values and thesecond group of word lines would be read with the second set of readcompare values.

FIG. 19 shows an example set of threshold voltage distributions that aresimilar to the distribution depicted in FIG. 9. FIG. 19 depicts two readcompare values for each state. Read compare values Vra1 and Vra2 areassociated with state A. Read compare values Vrb1 and Vrb2 areassociated with state B. Read compare values Vrc1 and Vrc2 areassociated with state C. In one embodiment, the first set of readcompare values includes Vra1, Vrb1 and Vrc1; the second set of readcompare values includes Vra2, Vrb2 and Vrc2; and Vra1>Vra2, Vrb1>Vrb2and Vrc1>Vrc2. Thus, when performing step 930, the process of FIG. 17 isperformed once for WL0 with Vcgr=Vra1, once for WL0 with Vcgr=Vrb1, andonce for WL0 with Vcgr=Vrc1. When performing step 932, the process ofFIG. 17 is performed once for each of the subject word lines withVcgr=Vra2, once for each of the subject word lines with Vcgr=Vrb2, andonce for each of the subject word lines with Vcgr=Vrc2. In otherembodiments, the first set of read compare values could include Vra1,Vrb1 and Vrc2; or the first set of read compare values could includeVra1, Vrb2 and Vrc2. Other permutations can also be implemented.

In another embodiment of step 930, the first set of read compare valuesis used to read a first subset of memory cells connected to word lineWL0 and a third set of read compare values is used to read a secondsubset of memory cells connected to word line WL0. The two subsets canbe memory cells in different planes or groupings as described above. Inone implementation, the two subsets are disjoint, for example, odd andeven bit lines; or the first plane or grouping includes memory cells onNAND strings connected to bit lines 0 through (½(x)-1) and the secondplane or grouping can includes memory cells on NAND strings connected tobit lines ½(x) through (x-1), where x is the total number of bit linesfor user data. Other groupings can also be used. Note that the third setof read compare values is different (partially or completely) than thesecond set of read compare values.

In another embodiment of step 930, the first set of read compare valuesis used to read data for a first page of data stored in the memory cellsconnected to word line WL0 and a third set of read compare values isused to read data for a second page of data stored in the memory cellsconnected to word line WL0. The third set of read compare values isdifferent (partially or completely) than the second set of read comparevalues.

FIG. 20 describes another embodiment of a process for operatingnon-volatile storage that includes using different read compare valuesfor a particular word line. In step 950, the memory cells connected to afirst set of word lines are programmed using the second set of targetlevels mentioned above. In step 952, the memory cells connected to aword line at the edge of a boosting region are programmed using thesecond set of target levels mentioned above. In step 954, the memorycells connected to the remaining word lines are programmed using thesecond set of target levels mentioned above. Note that the word line atthe edge of the boosting region is not part of the first set of wordlines and the remaining word lines.

During steps 950, 952 and 954 of FIG. 20, the programming operationsinclude providing a 0 volt signal (or other appropriate signal) on aword line or select line to turn off the corresponding transistors inorder to create the boosting region. One example includes applying 0volts to the source select line SGS so that the source select gate turnsoff, which cuts off the channel from the source line and help cause theboosting of the NAND string. In some embodiments, a word line connectedto the NAND string can receive a 0 volt signal (or other appropriatesignal) to cut off the memory cells connected to that word line so thatthe boosting region ends or starts at that word line. This can also beused to create multiple boosting regions.

In step 960, the memory cells connected to the first set of word linesare read using the second set of read compare values mentioned above. Instep 962, the memory cells connected to the word line at the edge of theboosting region are read using the first set of read compare valuesmentioned above. In step 964, the memory cells connected to theremaining word lines are read using the second set of read comparevalues. Note that the arrow between step 954 and 960 is dashed torepresent that step 960 may performed at a different time than step 954and/or in a manner unrelated to step 964.

In some embodiments of step 962, different sets of read compare valuescan be used to read different pages of data associated with the wordline at the edge of the boosting region. Different sets of read comparevalues can be used to read planes or groupings of memory cells connectedto the word line at the edge of the boosting region. In bothalternatives, the different sets of read compare values are alsodifferent (partially or completely) from the second set of read comparevalues.

Although the above discussion focuses on reducing the impact of programdisturb on particular word lines, the current invention can also be usedto reduce the impact of word lines that have wider Vt distributions forany other reason. One other reason for the occurrence of wider Vtdistributions on some word lines can be so-called over-programming dueto, for example, fast programming memory cells being concentrated on oneor a limited number of word lines. Over-programming also results insimilar Vt distributions as depicted in FIG. 12. By applying differentverify target levels or read compare values to such word lines, theimpact of the Vt distribution widening on those word lines can bereduced as well.

The foregoing detailed description of the invention has been presentedfor purposes of illustration and description. It is not intended to beexhaustive or to limit the invention to the precise form disclosed. Manymodifications and variations are possible in light of the aboveteaching. The described embodiments were chosen in order to best explainthe principles of the invention and its practical application to therebyenable others skilled in the art to best utilize the invention invarious embodiments and with various modifications as are suited to theparticular use contemplated. It is intended that the scope of theinvention be defined by the claims appended hereto.

1. A non-volatile storage system, comprising: non-volatile storageelements, said non-volatile storage elements include a set ofnon-volatile storage elements connected to a particular word line andgroup of non-volatile storage elements connected to a group of wordlines different than said particular word line; and a managing circuitin communication with said non-volatile storage elements, said managingcircuit programs said group of non-volatile storage elements using agroup of target levels, said managing circuit programs said set ofnon-volatile storage elements using a first set of target levels for afirst page of data and a different set of target levels for a secondpage of data, at least one of said first set of target levels isdifferent than a corresponding target level of said group of targetlevels, at least one of said different set of target levels is differentthan a corresponding target level of said group of target levels, saiddifferent set of target levels is different than said first set oftarget levels.
 2. A non-volatile storage system according to claim 1,wherein: said particular word line is adjacent to a source control line.3. A non-volatile storage system according to claim 1, wherein: saidnon-volatile storage elements are multi-state NAND flash memory devices.4. A non-volatile storage system according to claim 1, wherein: saidmanaging circuit reads said set of non-volatile storage elements using afirst read compare value and reads said group of non-volatile storageelements using said first read compare value.